Optical waveguide lasers are currently of considerable interest. For instance, single mode Er-doped optical fiber lasers are currently considered to be strong candidates for use as signal radiation sources in optical fiber communication systems. See, for instance, G. A. Ball et at., Optical Fiber Communication Conference, Feb. 2-7, 1992, San Jose, Calif., "1992 Technical Digest Series", Vol. 5 Conference Edition, p. 97. Pr-doped waveguide lasers and amplifiers are also of current interest.
Optical waveguide lasers potentially have advantages over semiconductor laser sources. For instance, the former inherently have a simple structure, can easily be produced to emit a predetermined wavelength, and can be readily continuously tuned. See, for instance, G. A. Ball et al., Optics Letters, Vol. 17(6), p. 420.
In view of the intrinsic simplicity of optical waveguide lasers, those skilled in the art had, up to now, not had any reason to doubt that an otherwise appropriate waveguide laser would have an output that is stable enough to meet the (very exacting) stability requirements for use as a signal radiation source in an optical communication system. Typically such systems are designed to achieve a bit error rate (BER) of less than 10.sup.-9, even less than 10.sup.-12. This implies the need for a very stable laser.
It is known that diode-pumped Nd: YAG lasers can be subject to relaxation oscillations, which add considerable noise to the laser output, typically in the frequency range 100 kHz-1 MHz. See T. J. Kane, IEEE Photonics Technology Letters, Vol. 2(4), p. 244. That paper reports the presence of a relaxation oscillation peak at 280 kHz in an exemplary diode pumped single frequency monolithic Nd:YAG laser. The paper also reports that provision of appropriate feedback can effectively remove the relaxation oscillation intensity noise from the output of the Nd:YAG laser, and FIG. 2 of the paper shows a block diagram including the feedback loop.
The luminescing species in Er-doped fiber amplifiers and lasers is Er.sup.3+. It is known that, in a SiO.sub.2 -based matrix, the relevant electron transition of Er.sup.3+ has a long lifetime, exemplarily about 10 ms.
Optical waveguide lasers advantageously comprise in-line refractive index gratings. See, for instance, U.S. Pat. No. 4,725,110. U.S. patent application Ser. No. 07/878,802, incorporated herein by reference, discloses an advantageous method of making such a grating.